Since an organic light-emitting device capable of high brightness light emission was invented by C. W. Tang et al. of Kodak in 1987 (see, Appl. Phys. Lett., Vol. 51, page 913 (1987)), the development of materials therefor and improvement of the device structure are abruptly proceeding and its practical use in a display for car audio, cellular phone and the like has been started in recent years. In order to further expand applications of this organic EL (electroluminescence), studies are being aggressively made to develop materials for enhancing the luminous efficiency or durability or to develop full-color display.
As for the luminous efficiency, since the ratio of exciton produced in the electrical excitation between the singlet excited state and the triplet excited state is 1:3, the internal quantum efficiency in light emission of an organic EL using, as the light-emitting material, a fluorescent material which emits from singlet excited state has an upper limit of 25% (see, Gekkan Display (Monthly Display), separate volume “Organic EL Display”, page 58 (October, 1998)). On the other hand, when a phosphorescent material emitting from triplet excited state is used as the light-emitting material, the produced singlet excited state and triplet excited state both contribute to the light emission and therefore, the upper limit of internal quantum efficiency is 100%.
Many of phosphorescent materials are a compound containing a heavy metal atom. Y. Ma et al. disclose an organic EL device using, as the light-emitting material, a coordination compound having gold as the center metal (see, Adv. Mater., Vol. 11, page 852 (1999)). Thereafter, V. W.-W. Yam et al. have also taken notice of good phosphorescent emission property of the gold complex and use it as the light-emitting material of an organic EL device (see, Chem. Commun., page 53 (2000)).
For production of an organic EL device, a vacuum vapor deposition method has been conventionally used. However, this method has problems in that vacuum equipment is necessary and that the larger the area of the device is, the more difficult it is to form an organic thin film having a uniform thickness. On the other hand, an inkjet method and a printing method, which are being developed as a technique of forming the film by coating, can form a film under atmospheric pressure and moreover, ensure excellent performance in the large-area formation or mass production of the device. In the film formation by these methods, a low molecular weight compound having possibility of causing phase separation or segregation cannot be used and therefore, a polymer light-emitting material of undergoing no crystallization must be used.
However, most compounds heretofore known as a polymer containing a gold complex have a structure such that gold is contained in the polymer main chain (see, Chem. Commun., page 1055 (1998)). Such a structure is defective in that the gold concentration in the polymer can be hardly controlled, that a multifunctional polymer having a light-emitting moiety and an electron transport moiety or the like can be hardly obtained, or that the molecule decomposes by dissociation of bond between gold and an organic group. Therefore, light-emitting material of an organic EL device is required to be a compound having a polymer main chain constituted only by an organic group.
As for the color of emitted light, studies are being aggressively made in recent years to develop a full color display or a white light source by utilizing an organic EL device and under these circumstances, the problem to be solved is to develop, among phosphorescent materials having a high luminous efficiency, a multi-color light-emitting material including blue light emission having a short wavelength.
In this way, for producing a large-area colored/white organic EL device having high luminous efficiency, development of a polymer material facilitated in the control of light emission color and containing a phosphorescent emission moiety is being demanded.